I just started using 4.0, and it does some strange things with rafts; it created a small raft under the points that touch the build plate, but left major sections of the part I was trying to print unsupported. I went back to 3.1 at least while using rafts.

S3D 4 installs into a different folder, so you can have both installed at the same time. I'm unsure if/how the profiles are separated, so I would make a full backup of the v3 just in case. I'm using 4 and haven't found a need to go back, but I still have 3 installed as a backup should the need arise.

I have noticed some bugs in 4.0 in creating separate print processes within a single object (you could do this anyway in the previous version):

1. S3D will not recognize top and bottom layers in between separate print processes....it only recognizes the top and bottom of the entire object.

For example, if I have a 4.00 mm tall cube and split it into 2 processes:
0.00mm to 2.00mm with 2 top and bottom layers @ 0.20 mm layer height
2.00mm to 4.00mm with 3 top and bottom layers @ 0.20 mm layer height

It will just do 2 bottom layers at the bottom of the object and 3 layers at the top of the object....it did not do this in the previous version.

2. You can't adjust 1st layer height with different processes.

Same as last example, I can't modify that 1st layer at 2.00 mm height in the process.

The variable settings wizard isn't much help either as it crashes constantly regardless of object...and it is essentially doing what I just described but with reduced functionality from the previous version. Overall this a big step backwards in customization of model printing which is disappointing.